a. Field of the Invention
The present invention relates to a joiner for clinch joining ductile materials, such as metal sheets, and in particular to a joiner including a die and a punch assembly.
b. Related Art
It is known to join a plurality of sheets of ductile material by causing these to be deformed into an interlocking configuration in a local area. Such joins are made by ductile material joining tools comprising a die with an aperture that is opposite a punch assembly comprising a punch surrounded by a stripper mechanism. Layers of ductile material are sandwiched between the punch assembly and when the punch is pressed towards the aperture, material is drawn into the aperture. The material undergoes plastic deformation in the aperture to flow into a shape in which two or more layers are interlocked, for example by the forming of one layer around another layer.
The aperture has a base with an anvil having an anvil surface and at least two side walls formed from movable blades. The blades are generally transverse to the anvil surface and extend in the direction in which the die and punch are pressed together. The blades help define the local area, for example a circular, square or rectangular area, in which the deformation of the layers of sheet material takes place. Once the material has been drawn and flows into the aperture, the blades move away from each other in a radial direction as sheet material flows laterally.
The outward movement of the blades is constrained by a die shield, which may be separate from but held in a fixed relationship with the die. In order to provide a compact die, the die shield can be joined to or integral with the die.
A circular die and punch can be used to form a clinch joint in which sheet material is symmetrically deformed both axially and radially to form a leak-proof button, for example as disclosed in patent document U.S. Pat. No. 5,150,513. A rectangular die and punch can be used to form a trapezoidal clinch joint (also called a lance joint), in which the sheet material is cut through by the punch along a pair of parallel opposed lines, with the layers of sheet material deformed laterally outwards underneath each of the cuts.
One commercially available example of a joining tool for forming a circular clinch joint is the SR 504 Series die and punch tool set from Bollhoff Fastenings Limited of Willenhall, West Midlands, UK. This tool set has a nominal 5 mm aperture with four movable die blades constrained by a separate die shield having an inner diameter of 16 mm.
A more compact die and punch would be desirable, for example allowing the sheet material joining tool to get further into corners or other awkward locations when fabricating a structure from the sheet material. However, there is a trade off between the strength of the tool and the maximum thickness of sheet material that may be joined, and the overall size, particularly in the direction transverse to the direction in which pressure is applied.
The die shield is arranged so that much of the pressure between the die and punch assembly is born by the die shield and stripper mechanism. However, the die blades experience increasing pressure as material is pressed into the aperture. This pressure can result in restriction of the movement of the die blades, resulting in a bad joint and/or damage to the die blades, particularly if the die blades are made thinner to reduce the lateral extent of the die.
A further constraint results from the necessity to include in the die some means of biasing the die blade back towards the anvil surface after the drawing operation by the punch is completed. For this, a coil spring or o-ring can be provided extending around the outside of the die blades. As the die blades move outwards to dilate the aperture, the spring or o-ring becomes stretched or compressed. When the joined sheet material is withdrawn from the aperture, the die blades return to their start position owing to the tension or compression in the spring or o-ring.
Because the spring or o-ring extends around the outside of the die blades usually between the die blades and the surrounding die shield, lateral space must be provided for the spring or o-ring. This again limits further reduction in the lateral extent of the die. Furthermore, lateral clearance space can result in a die blade being dislodged from between the anvil and die shield, and being lost from the die. This is very inconvenient in a production environment, as any machine using the sheet metal joiner would then have to be stopped to repair or replace the faulty die. If the faulty die were not spotted immediately, a great deal of rework to joined fabrications might then be required.
In some dies, the die blades slide laterally outwards, in which case the aperture depth is unchanged as the die blades move. However, the pressure imparted on the die blades can inhibit such a sliding motion, unless a die shield is used. Such a die shield will increase the lateral extent of the die.
Sometimes the die blades pivot outwards about a pivot mechanism below the level of the anvil surface. The pivot mechanism has a pivot axis or pivot point below and laterally outside an edge of the anvil surface. Because of this and the requirement that die blades should have a wide base or pivot with sufficient surface area to withstand the pressures during drawing, the die blades tend to rise when pivoted outwards. This tends to increase the pressure on the die blades, and again limits the amount by which the lateral dimensions of the die can be reduced.
The present invention addresses the problems cited above, and provides a die for a ductile material joiner, and also a ductile material joiner for clinch joining two or more layers of ductile material, which addresses these issues.
Accordingly, the invention provides a die for a ductile material clinch joiner, comprising:
a) a die anvil, the anvil having a body portion and an anvil surface;
b) at least two die blades, the blades extending generally transverse above and below the anvil surface and forming with the anvil surface a die aperture for a die punch, the separation between the blades defining a die aperture width or diameter and the extension of the die blades above the anvil surface defining a die aperture depth;
c) at least one pivot recess in the body portion;
d) a protrusion on each of the die blades, each protrusion being seated in a matching pivot recess to form with the recess a pivot joint by which each die blade may pivot with respect to the anvil body portion in order to constrict and dilate the aperture width or diameter;
e) at least one biasing member by which the die blades are biased to constrict the die aperture; and
f) a die shield around the die blades;
wherein the pivot joints extend underneath the anvil surface so that when the die aperture is dilated the aperture depth is decreased.
Also according to the invention, there is provided a ductile material joiner for clinch joining two or more layers of ductile material, comprising a punch tip and a die according to the invention, the die having an aperture matching the punch tip.
In a round clinch joint, the punch tip should match the die aperture with clearance that is sufficient so that ductile material is drawn down between the punch tip and die blades prior to compression of the material against the anvil surface and consequent lateral flow of the ductile material.
In a lance joint, the punch tip should match the die aperture with a close clearance so that ductile material is cut along the die blades prior to compression of the material against the anvil surface and consequent lateral flow of the ductile material under the cut.
The provision of the pivot joint underneath the anvil surface can be used to reduce the lateral extent of the die blade. This is because the joint then extends laterally further inwards, for example towards a central longitudinal axis of symmetry of the die.
At the same time, this arrangement permits the depth of the aperture to be decreased when the aperture is dilated, and this helps to permit a reduction in pressures borne by the die blades as the die blades move outwards during the joining of the sheet material. This facilitates movement of the die blades and hence formation of the joint. For example, the pivot joint will have a pivot point or pivot axis, and provision of at least a part of the pivot joint underneath the anvil surface permitting this axis to be moved inwards. Bringing the pivot axis laterally inwards allows the die blade to pivot so that as the aperture dilates, the depth of the aperture decreases.
It is not necessary however, for the aperture depth to decrease as the die blades first start to pivot laterally outwards. For example, the pivot point or pivot axis may be laterally between a pair of longitudinal axes defined by the limits of travel of tips of the dies blades extending above a substantially flat anvil surface, the pair of longitudinal axes being transverse to the anvil surface. Then, the height of the die blade tips above a plane defined by the anvil surface will fall as the die blade become fully dilated. If the pivot point or pivot axis is approximately central between the pair of longitudinal axes, then this height will initially rise as the die blades begin to move laterally outwards, then fall as the die blades move fully outwards. Preferably, the pivot point or axis will be closer to the innermost of the pair of longitudinal axes, so that height of the tip of the die blade above the plane defined by the anvil surface is lower when the aperture is fully dilated than when the aperture is constricted. In any event, depth of the aperture may increase as the aperture is partially dilated, and then decrease as the aperture is fully dilated.
The pressure on the die blade is then partially relieved during the time when a maximum pressure would be expected to be exerted on the die blades, i.e. as the sheet material joint forming process completes formation of the joint.
The recess underneath the anvil surface may be a part spherical or a part cylindrical pivot socket, in which case the die blade protrusion may be a convex surface matching the pivot socket so that the convex surface rotates in the pivot socket when the die blade is pivoted. The surfaces may be essentially spherical, with clearance provided for manufacturing tolerances. Additional clearance may be required if the pivot joint is not cylindrical. For example, the pivot joint can be part-toroidal with the blade protrusion having a similar part-toroidal shape. Additional clearance must then be provided between the protrusion and the recess to allow for the fact that as the protrusion pivots in the recess, the central part and end parts along the arc of the protrusion will move relatively apart in a longitudinal direction.
Preferably, the anvil has a shoulder that extends in a plane below the anvil surface and that supports a die blade endmost portion. The die blade endmost portion may be a portion of the die blade protrusion. In a preferred embodiment of the invention, the shoulder extends tangentially from the pivot recess.
It is advantageous if the biasing member is provided between the die blade and the shoulder. The biasing member then does not add to the lateral extent of the laterally biased die blades. The biasing member is preferably a resilient ring, for example a nitrile o-ring, that is compressed between the die blade and the shoulder when the die blade is pivoted to dilate the aperture.
There may however, be alternatively or additionally a resilient biasing member, for example a coil spring or an o-ring, stretched around laterally outwards facing surfaces of the die blades below the level of the anvil surface. The effect of a biasing member provided between the die blade and the shoulder may permit a reduction in the size of a second biasing member stretched around laterally outwards facing surfaces of the die blades. This facilitates a reduction in the lateral extent of the die, whilst at the same time maintaining good inward biasing of the die blades.
In one embodiment of the invention, the anvil surface is bounded by a generally circular periphery, the die blades extending in an arc around at least part of said periphery.
In an alternative embodiment of the invention, the anvil surface is bounded by a generally square or rectangular periphery, the die blades extending along at least two opposed sides of said periphery.
It is advantageous if the die shield remains in close proximity with a portion of the die blade adjacent the protrusion at the die blade is pivoted to dilate and constrict the aperture. This helps to keep each die blade retained to the die.
In one embodiment, the die shield is integral with the body portion, and the die blades are removably inserted in an opening in a side of the body portion. In another embodiment of the invention, the die shield is removably attached to the body portion and the die shield prevents the die blades from being removed from the die when the die shield is attached to the body portion.